[Ksummit-discuss] [CORE TOPIC] More useful types in the linux kernel

[Dan Carpenter]

On Fri, Aug 12, 2016 at 10:38:35AM +1000, NeilBrown wrote:
> On Fri, Aug 12 2016, Dan Carpenter wrote:
> 
> > On Fri, Jul 22, 2016 at 03:57:40PM +0200, Hannes Reinecke wrote:
> >> > 
> >> > I guess that almost all functions return only a few possible error codes?
> >> 
> >> Precisely. If we had a way of specifying "the return value is an errno
> >> with the possible values '0', '-EIO', and '-EINVAL'" that would be
> >> _so_ cool.
> >
> > I think that's a bad idea.  We should be propagating errors from the
> > functions we call.  It should be able to change without breaking.
> 
> Should we?  I recently faced a bug caused by a (proposed) change to
> btrfs which returned a different error code to the ->fh_to_dentry()
> function. That was being propagated up to nfsd, and out on the wire in
> the NFSv4 protocol.
> Only the new error was invalid for the protocol and the client
> (correctly) reported it to user-space rather than handling it
> internally.
> 
> This happened because not enough thought/documentation had been given to
> which error codes were sensibly meaningful.  I changed
> exportfs_decode_fh() so that any error other than ENOMEM became ESTALE,
> because that is all nfsd can sensibly handle.
> 
> I'm sure there are some (many!) paths where error codes should be
> propagated transparently, but I don't think we should assume that is
> always true.
> 

My guess is that 97% of the time it should be propagated.

> >
> > Smatch does a pretty good job of tracking return values, especially
> > if you rebuild the database over and over once a day like I do.
> 
> So it is OK to keep a list of valid return values in a database, but not
> OK to keep them in the code as documentation, and to alert the
> programmer when they make a change so they can declare (and maybe even
> document) if it was an intentional change?

What I'm saying is that no one wants a thing where if we change a
return code, we have to update the documentation for all the call
trees.

regards,
dan carpenter